Conventional rapidly tunable lasers usually contain a laser medium that amplifies over a wide range of wavelengths, and a tunable optical bandpass filter. When the frequency to which the optical bandpass filter is tuned is increased, it can occur that the laser activity stops, because the light in the resonator can no longer perform enough round trips to develop saturated laser activity before the optical bandpass filter is tuned to a new wavelength. This problem can only be solved to a limited degree by using resonators of shorter length, since shorter resonator lengths increase the laser's mode spacing and give rise to increased noise.
In order to overcome this problem, document WO 2006/079078 A3 discloses the concept of spectral mode locking, or “Fourier Domain Mode Locking” (FDML). In accordance with this concept, the optical filter is tuned synchronously with the round trip time of the light in the resonator. This means that light of a specific wavelength hits the optical filter again after one round trip in the resonator when the optical filter is again tuned to this particular wavelength. FDML lasers have the advantage over conventional rapidly tunable lasers that there is no fundamental restriction on the tuning speed, and that they have a narrower line width and a greater coherence length. They also offer lower noise and greater power than conventional rapidly tunable lasers, and do not generally have a discrete mode structure.
One disadvantage of known FDML lasers, however, is that the synchronization of the round trip time in the resonator with the filtered tuning time requires the use of a very long optical delay line. A very long glass fiber is usually used as the delay line. If the optical filter is, for instance, tuned at a frequency of 30 kHz, the glass fiber must be 7 km in length in order to match the round trip time in the delay line to the filter's tuning frequency. Relatively high costs are incurred by the use of such long glass fibers. In addition, the long glass fiber means that this kind of FDML laser is relatively large.